It is known to provide filter cartridges with concentric filtering chambers, as disclosed, for example, in U.S. Pat. No. 3,791,105. Due to the concentric arrangement of the filtering chambers for the individual filtering materials and the consequent labyrinth-like flow pattern in the filtering device, the distance to which the filtrate is forced to travel is much longer than the distance in filter types in which each individual filtering medium extends over the entire cross-sectional area of the filter cartridge. It is well known that the longer the filtering distances the better is the resulting filtering effect.
The above-mentioned U.S. patent discloses a filtering device providing a two-stage filtering of a gaseous fluid. Even though the concentrically mounted chambers, in that device, accommodate filtering media of different materials and of at least different densities, both filtering stages provide only oil removal from the gaseous fluid.
In U.S. Pat. No. 3,464,186, there is disclosed a dryer for compressed fluid, such as compressed air in an air brake system for a vehicle, which is in the form of two chambers one containing stainless steel mesh acting as both a coarse oil condensing and water condensing medium and the other a desiccant for absorbing water vapor. In U.S. Pat. No. 3,353,339, there is disclosed a straight-through filter through which air flows across a porous hydrophobic membrane removing entrained liquid water and dirt, across a disposable canister filled with an adsorbent medium, such as activated carbon, to remove oil vapor and oil mist, and a filter pad to remove solid fines which may have been picked up during flow through the filter. U.S. Pat. No. 1,896,833 discloses an oil and water extractor having a first chamber containing metal shavings on which liquid oil and liquid water collect and pass by gravity into a lower sediment collecting portion, a second chamber provided with baffles for collecting residual liquid particles, and a third chamber containing a highly porous filtering material. U.S. Pat. No. 4,015,969 discloses an in-line filter for gases, such as air, using two coaxial chambers, the outer chamber containing a coarse filter, such as stainless steel wool to condensate vapors, both oil and water, the drops of liquid falling at the bottom of the filter housing, and a central chamber containing a desiccant.
None of the known prior art, however, is successful in completely removing from a gaseous fluid, such as breathable compressed air, oil mist and water vapor. This is due to the fact that none of the prior art suggest the critical sequence of steps consisting in first removing the majority of the water vapor from the compressed air, which permits the second step of oil removal to be efficiently accomplished, followed by the third step of removing the remaining water vapor.
In many instances, the gaseous fluid to be filtered, especially breathable compressed air, also contain a considerable portion of water vapor. This water vapor, however, renders the oil removal process rather difficult. Consequently, the oil removal characteristics of the prior art filtering devices is relatively poor, such that the air that has been subjected to oil removal in the prior art filtering devices is not suitable, for instance, for breathing purpose. Furthermore, the relatively high content of water vapor that remains in the filtered air is undesirable for many other applications.
It is recognized that filter devices have been known in the past in which different filtering media are positioned sequentially in the direction of fluid flow, and which are also provided with drying means in addition to oil-removal means. However, in these prior art filtering devices, the individual filtering media are always extending over the entire cross-section of the filter cartridge, which results in relatively short filtering travel distances through each filtering medium. Consequently, the filtering effect of those prior art filtering devices is relatively poor. Therefore, adequate oil removal and de-humidification of the filtrate to make it suitable for breathing is not assured with these prior art filtering devices.